netdev - Re: [RFT PATCH 3/9] net: ethtool: break association of ETH_FLAG_* with NETIF_F

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Date:	Mon, 20 Jun 2011 13:51:09 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	Michał Mirosław <mirq-linux@...e.qmqm.pl>,
	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>
Subject: Re: [RFT PATCH 3/9] net: ethtool: break association of ETH_FLAG_*
 with NETIF_F_*

On Mon, 20 Jun 2011 21:11:21 +0100
Ben Hutchings <bhutchings@...arflare.com> wrote:

> On Mon, 2011-06-20 at 21:14 +0200, Michał Mirosław wrote:
> > This is the only place where NETIF_F_* feature flags are exposed
> > to userspace.
> 
> Except sysfs.
> 
> > After this patch feature flags may be changed/reordered freely.
> [...]
> 
> Really, what do you think was the point of exposing features through
> sysfs if they are going to be changed?
> 
> Oh, but they have been changed already:
> 
> v2.5.70
> net-sysfs added
> 
> v2.6.15
> +#define NETIF_F_UFO             8192    /* Can offload UDP Large Send*/
> 
> v2.6.18
> -#define NETIF_F_TSO            2048    /* Can offload TCP/IP segmentation */
> -#define NETIF_F_UFO             8192    /* Can offload UDP Large Send*/
> +#define NETIF_F_GSO            2048    /* Enable software GSO. */
> +#define NETIF_F_GSO_SHIFT      16
> +#define NETIF_F_TSO            (SKB_GSO_TCPV4 << NETIF_F_GSO_SHIFT)
> +#define NETIF_F_UFO            (SKB_GSO_UDP << NETIF_F_GSO_SHIFT)
> +#define NETIF_F_GSO_ROBUST     (SKB_GSO_DODGY << NETIF_F_GSO_SHIFT)
> +#define NETIF_F_TSO_ECN                (SKB_GSO_TCP_ECN << NETIF_F_GSO_SHIFT)
> +#define NETIF_F_TSO6           (SKB_GSO_TCPV6 << NETIF_F_GSO_SHIFT)
> 
> v2.6.23
> +#define NETIF_F_IPV6_CSUM      16      /* Can checksum TCP/UDP over IPV6 */
> +#define NETIF_F_MULTI_QUEUE    16384   /* Has multiple TX/RX queues */
> 
> v2.6.24
> +#define NETIF_F_NETNS_LOCAL    8192    /* Does not change network namespaces */
> 
> v2.6.27
> -#define NETIF_F_MULTI_QUEUE    16384   /* Has multiple TX/RX queues */
> 
> v2.6.29
> +#define NETIF_F_GRO            16384   /* Generic receive offload */
> 
> (I've omitted changes that use previously unused bits.)
> 
> If we're going to keep changing features (maybe even more often) then we
> have to do something about this sysfs attribute.  Maybe get rid of it
> (as it seems not to be widely used, thankfully).  Maybe fix it to use
> the same feature values as today, but no new features.  But certainly
> don't pretend that feature flags are not exposed.
> 
> Ben.
> 


I have no problem with dropping or changing the sysfs feature output.
It is useful to have a way to check if device supports something. Sysfs
predates the feature support bits of ethtool. As long as ethtool supports
it then I am fine with that.

It would be nice to have a non-grotty way of doing something like:
  if ethtool eth0 --supports gro 
  then
       ...
  fi

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